The invention relates to an ignition device and to a method for producing it.
Spark plugs essentially comprise a metal housing and an insert, which includes a ceramic insulator, a connection bolt, and a middle electrode. The steel materials used for the housing, for reasons of function, production and cost, corrode above all from the hot, corrosive atmosphere of the kind that occurs during operation in a vehicle. To protect against corrosion, the steel housing is typically coated with a metal layer.
As the metallizing layer, zinc or nickel is used, for instance. The anti-corrosion action of the zinc is based on the fact that as a less-noble metal, it corrodes instead of the iron and forms so-called white rust. This cathodic protection effectively prevents the iron from corroding. The formation of white rust is undesirable, however, from an aesthetic standpoint.
Nickel, which is a more-noble metal than iron electrochemically, can also be used to create an anti-corrosion layer. A problem then is that mechanical injuries to the protective layer lead to corrosion of the exposed iron and hence lead to the formation of so-called red rust. In German Patent DE-PS 38 41 215 C2, it is proposed that to prevent rust formation a chromate layer be applied to the nickel protective layer; the chromate layer covers the cracks and pores in the protective layer. Chromate treatment involves major environmental risks, however.
The ignition device of the invention has the advantage that the paint layer brings about effective corrosion protection that is durable even at elevated temperatures and is simple to achieve. For instance, a spark plug that has been provided with a paint coating according to the invention exhibits no evidence of corrosion whatever even after a 100-hour salt-spray test.
With the provisions recited in the dependent claims, advantageous refinements of the ignition device according to the invention and the method according to the invention are possible.
For instance, the painting of the ignition device can be combined with other methods for corrosion protection, such as nickel-plating or zinc-plating. Furthermore, many painting methods are available that make it possible to adapt the painting process to the assembly of the ignition device. The painting of the individual components is preferably done after they have been mounted on the finished ignition device.
It can be considered especially advantageous that with the aid of a paint coating, beyond the corrosion protection not only can the visual appearance of the ignition device be improved but the sliding property, for instance, of a thread stamped onto the hous ing is also favorable affected.